4,4'-Dihydroxydiphenylsulfone (hereinafter referred to as "4,4'-compound") is excellent in resistance to heat and oxidation, stability to light, etc. and has therefore found wide use in recent years in the field of the high polymer industry relating to polyester resin, epoxy resin, polycarbonate resin, polyether sulfone resin and the like. When high-polymer products are prepared from 4,4'-compound containing impurities such as 2,4'-dihydroxydiphenylsulfone (hereinafter referred to as "2,4'-compound") and trihydroxytriphenyldisulfone (hereinafter referred to as "tri-compound"), the products tend to have a decreased molecular weight and impaired mechanical properties. This tendency becomes more pronounced when 2,4'-compound, tri-compound and like impurities are present in larger quantities. Accordingly, it has been desired to supply 4,4'-compound of high purity free from the impurities such as 2,4'-compound and tri-compound to the greatest possible extent 4,4'-compound is useful also as a material for couplers for color photography, as a color developer for heat-sensitive recording paper, etc., and a high purity is also desirable in such uses.
Industrially, 4,4'-compound is prepared primarily by the dehydration reaction of phenol with a sulfonating agent such as sulfuric acid. The reaction mixture contains 2,4'-compound, tri-compound and like sulfones, other sulfonic acids, etc. as impurities. When sulfonic acids, etc. are removed from the reaction mixture, the resulting crude 4,4'-compound usually contains about 70 to about 80 wt. % of 4,4'-compound.
Processes have been developed for preparing 4,4'-compound of high purity by inhibiting formation of 2,4'-compound. For example, a process for producing 4,4'-compound of high purity is proposed wherein 4,4'-compound is produced by a dehydration reaction while effecting precipitation of the product and isomerization of 2,4'-compound, a by-product, to 4,4'-compound (U.S. Pat. No. 4,162,270). The crude 4,4'-compound obtained by removing sulfonic acids and the like from the reaction mixture of this process usually contains about 90 to about 95 wt. %. Nevertheless, it is desired that the 4,4'-compound for the various uses mentioned above be at least about 97 wt. % in purity, for example, because higher qualities are required of resins in recent years. Accordingly, the compound prepared by the former process must be further purified, and the compound obtained by the latter process should preferably be further purified.
Processes for purifying 4,4'-compound have been proposed which employ various organic solvent mixtures, such as o-dichlorobenzene-phenol (Examined Japanese Patent Publication SHO 51-36264), o-dichlorobenzene-acetic acid (Examined Japanese Patent Publication SHO 57-48152), o-dichlorobenzene-ethyl acetate (Examined Japanese Patent Publication SHO 57-48153) and o-dichlorobenzene-alcohol (Examined Japanese Patent Publication SHO 58-2234). However, these processes all use organic solvents and accordingly have the problem that the solvents are inconvenient to handle and hazardous from the viewpoint of sanitation and cause environmental pollution.
Alternatively without using organic solvents, 4,4'-compound is usually purified by dissolving crude 4,4'-compound in an aqueous solution of sodium hydroxide or like alkali, treating the solution with active carbon and thereafter adding an acid, such as sulfuric acid, to the solution in an amount at least sufficient to neutralize the solution to precipitate 4,4'-compound This process utilizes the properties of 4,4'-compound that it is sparingly soluble in water but readily soluble in aqueous solutions of basic substances in the form of a mono- or di-metal salt of the compound. This process removes sulfonic acids but almost fails to remove 2,4'-compound, tri-compound and like sulfones, giving a product of a low purity which is in no way as high as is desired as will be apparent from the comparative example given later.